#2 Can't have road segments with different direction

If two road segments share a control point they can't
both have the shared control point as the start or end
point. If one of the segments has it as its start
point, the other segment must consider it its end
point. That it, the road segments must have the same
direction. This is because calculations during
rendering/serializing requires this. This should
therefor be fixed by enforcing this requirement when
road segments are created.